We have derived ultrastructural and cytochemical criteria that permit consistent identification of fiber types in normal adult skeletal muscle, and these now include immunological differences. Our recent emphasis has been on structural heterogeneity and the relationship to known biochemical and physiological characteristics in mammalian muscles. The immunocytochemical approach permits direct localization of specific myofibrillar proteins with respect to fiber type, and thus it may be posible to determine whether biochemical differences in the protein composition of whole muscles are related to differences in the pattern of distribution of fiber types. We shall continue to use antibodies directed against myosin and the proteolytic subfragments of myosin as cytochemical reagents to analyze these differences, and these studies will include ultrastructural as well as light microscopic observations. Fiber populations will be examined in mammalian skeletal muscles in well-defined functional states including postnatal development and denervation, where biochemical and physiological data have already been obtained in other laboratories. BIBLIOGRAPHIC REFERENCE: Gauthier, G.F. and S. Lowey (1975). Immunocytochemical localization of myosin among skeletal muscle fibers. J. Cell Biol., 67:131a. (Presented at the 15th Annual Meeting of the American Society for Cell Biology, November, 1975).